The present invention relates to a setter for burning, on which ceramic electronic parts are placed in a furnace when these ceramic electronic parts such as ceramic substrates and the like are burned, and particularly to a technique effective in suppressing each warp of multi-layer ceramic electronic parts burned.
In Japanese Patent Laid-open No. 2-74567, such a structure has been disclosed that since friction against a setter causes drawbacks of dispersion in the shrinkage rate and cracks and the like of a ceramic substrate in a process of burning the ceramic substrate, the ceramic substrate is floated by air during burning in order to prevent such drawbacks from be caused.
In such the structure, a microporous setter is always placed on an air intake auxiliary setter provided with air intake holes. Under this condition, the ceramic substrate is placed on the microporous setter and burned. During burning, air is supplied from the air intake holes of the air intake auxiliary setter and blows from micro-holes of the microporous setter toward a lower surface of the ceramic substrate and thereby causes the ceramic substrate to be floated.
In this structure, since the ceramic substrate is made apart from the setter during burning, the drawbacks such as shrinkage, cracks and the like of the ceramic substrate in burning are greatly suppressed, which have arisen previously.
In Japanese Patent Laid-open No. 11-79853, such a means is disclosed as to prevent contamination of burned products, which is caused by contact failure between the burned products and a setter during burning of electric material parts such as ceramic multi-layers or the like, and to prevent occurrence of inferior goods caused by adhesion between the burned products and the setter.
The above-mentioned means discloses a tape-forming method using a doctor blade method or the like so as to be able to manufacture thin setters by utilizing slurry, and the cited reference discloses a structure of the thin setter whose thickness is set to be 0.2 to 2 mm from a tape-forming point of view. The setter discloses a structure in which independent through holes are provided by a method like punching or the like of each contact surface between the burned products and the setter in order to be able to situate electric material parts such as ceramic substrates in nearly point contact manner. The setter is formed in a thin mesh shape, in which opening portions piercing a thin plate are provided.
From a point contact point of view, there is description in which one of the independent through holes has preferably an opening area set to be within a range of 0.07 to 36 mm2. It is also described that each opening ratio of the through holes is preferably within a range of 10 to 60% from a viewpoint of adhesive possibility between the setter and each through hole and of facility of a process of punching the through holes. Further, it is described that sintered density of the setter is set to be preferably 95% or more from a viewpoint of the strength the setter.
Japanese Patent Laid-open No. 5-270926 discloses a manufacture of lightweight setters having small thermal capacity by vesicating and hardening in a mold a mixture adding a polyurethane foam raw material to a mixture made of a ceramic powder and water, and thereafter by burning the mixture.
However, such a problem is pointed out that if ceramic electronic parts such as ceramic substrates and the like are situated on the setter having the above-mentioned structure and are burned, then characteristics of the ceramic electronic parts are deteriorated. As a means for solving the problem, from a viewpoint of replacement facility of peripheral atmospheres surrounding ceramic molds, the strength thereof and the like, it is disclosed that each ceramic mold is provided with one to ten through holes per 1 cm2 of each placement area thereof.
Japanese Patent Laid-open No. 6-281359 discloses such a structure that a setter on which each of electric material parts such as ceramic substrates and the like are situated and burned is provided with an uneven surface placement portion for situating the electric material parts such as ceramic substrates.
Japanese Patent Laid-open No. 5-267010 discloses a structure of a pot, which is used during manufacture of voltage nonlinear resistance members primarily containing zinc oxide, composed of a pot body provided with an opening portion therein and a separate porous plate setter provided on this opening portion. Therefore, the cited reference discloses that, even in the case where a plurality of pots formed by situating the voltage nonlinear resistance members on the porous plate setter is piled up in a furnace and is burned, dispersion of a varistor characteristic greatly affected by a burning atmosphere is decreased by making the burning atmosphere per pot as uniform as possible.
As mentioned above, various structures have been proposed about a ceramic setter for putting the ceramic electronic parts in a furnace, in order to suppress the occurrence of warp or the like of the ceramic electronic parts burned to the utmost. However, at present, the warp problem of the burned ceramic electronic parts has not been sufficiently solved yet. For example, although solution of the warp problem is effectively improved by making the opening ratio increase, the strength of the setter is lowered and if heat and cool treatment is repeatedly performed in the furnace, it is easy for the setter itself to warp. This is reflected to each quality of the ceramic electronic parts resultantly.
The ceramic setter is preferably considered to be burned without having an interaction between the ceramic setter and the ceramic electronic parts situated thereon, that is, giving any affect to the ceramic electronic parts. It is preferable that the ceramic electronic parts are simply situated so that burning behavior such as shrinkage based on the characteristics proper to the ceramic electronic parts is executed.
An object of the present invention is to provide a setter for burning, which can ensure the burning behavior based on characteristics proper to ceramic electronic parts, particularly, ceramic electronic parts having a multi-layer structure such as chip condensers, high frequency module parts and the like.
The above-mentioned and other objects and novel characteristics of the present invention will be apparent from the description of this specification and the accompanied drawings.
When the present inventors develop satisfactorily ideal setters for burning, they have observed in detail each warp amount of ceramic electronic parts burned by utilizing both a conventionally used typical setter for burning and a satisfactorily ideal one. And the inventors have tried to solve problems based on observed results.
Additionally, the inventors have thought that, in a relation between the setter for burning and the ceramic electronic part situated thereon, it is necessary to determine what evaluation standards the proper setter for burning should satisfy.
According to conventional evaluation standards, it is ideal that the warp amount of the burned ceramic electronic part is zero, and the validity of a ceramic setter is determined in comparison with the magnitude of each warp amount at a zero standard. However, the inventors have thought that it is necessary to determine the warp amount by using as a standard such a state that the ceramic electronic part is burned without being affected from the setter.
This is taken up as a large problem of a warp caused after the conventional ceramic electronic part is burned, and various proposals have been made, for example, by an improvement or the like in electroconductive paste as problems of the ceramic electronic part itself.
However, even if technology capable of achieving the warp state of zero in the burned ceramic electronic part itself is developed, in the case where the warp of the ceramic electronic part burned is caused by being affected from the setter at a burning stage thereof like the conventional setter, search of the cause for solving the warp state of zero of the ceramic electronic part burned complicates. Therefore, it is sufficiently thought that there arises such apprehension that the valid technology can not be developed as desired.
For this, the inventors speedily promote the technology development for achieving the warp state of zero of the ceramic electronic part burned by eliminating the interaction between the ceramic electronic part and the setter for burning to the utmost, and have thought that it is necessary to reexamine the evaluation standard of the setter for burning from a viewpoint of no influence on the burning behavior which the ceramic electronic part possesses originally.
The inventors have evaluated a valid property of various ceramic electronic parts by using as a evaluation standard the warp state of the ceramic electronic part burned, and thereby have developed a setter which affects a little the ceramic electronic part to be situated thereon.
According to the present invention, a setter for burning, which a ceramic electronic part is situated in a burning furnace when said ceramic electronic part is burned, comprises: an external wall; an electronic part placement surface for situating said ceramic electronic part thereon; and an in-furnace placement surface for situating said setter for burning in said burning furnace, wherein said setter for burning has a honeycomb structure in which a plurality of ventilation cells that is formed by each partition wall having a thickness of 0.05 to 1.0 mm and that are each partitioned in a pitch of 0.5 to 5.0 mm passes through between said electronic part placement surface and said in-furnace placement surface.
In this case, for example, a ceramic multi-layer substrate formed by piling up and laminating a plurality of green sheets each having a thickness of several xcexcm to several tens in is given as the above-mentioned ceramic electronic part. However, the above-mentioned ceramic electronic part does not require being limited to the ceramic multi-layer substrate, and, needless to say, may be a ceramic mold other than it. Moreover, even if being a metal, if being a mold for burning, which consists of powder thereof and organic materials, then the above-mentioned setter for burning, which is the present invention, can be utilized effectively.
The setter for burning, which has the above-mentioned structure, may have a honeycomb structure, for example, by forming such a plate-shaped member having an external wall of an almost square that a front surface thereof is a electronic part placement surface for situating the ceramic electronic part thereon and that a rear surface is an in-furnace placement surface, and by providing through holes capable of being ventilated from the front surface toward the rear surface to form ventilation cells.
In a burning furnace, a binder and the like included in the ceramic electronic part are burned, converted into gases such as CO, CO2 and H2O, and scattered. In the state that the ceramic electronic part is situated on the setter for burning, an external surface of the ceramic electronic part except for a surface of a side situated on the setter for burning becomes a free plane to which oxygen required for the scatter of the gases and for the combustion of the binder is supplied relatively freely.
However, in view of the supply of oxygen and the scatter of gases, oxygen can be supplied through the ventilation cells or the gases can be scattered through the ventilation cells, even in a side of the in-furnace placement surface of the setter for burning, which is easy transferred into a restricted state in comparison with the above-mentioned free plane. Therefore, in comparison with the case where the above-mentioned constitution is not provided, gasses of the binder or the like from the ceramic electronic part are scattered on average, so that an influence on shrinkage of the ceramic electronic part can be suppressed due to scattering on average.
As described above, because the ventilation cells strongly affect the shrinkage of the ceramic electronic part, situations for setting the ventilation cells have been examined.
For example, as shown in FIG. 8A, the ventilation cells may be constituted such that the adjacent cells thereof are partitioned by partition walls each having a thickness of 0.05 to 1.0 mm in at least three directions from a crossing section of three partition walls. FIGS. 8B and 8C illustrate the cases where four and six directions are partitioned, respectively. Additionally, a pitch between the adjacent ventilation cells may be set to be 0.5 to 5.0 mm. The cell pitch in these cases means a distance between the respective centers of the partition walls. For various cell shapes, the cell pitch may be defined as a distance between the partition walls corresponding to respective cells. Or a distance between the centers of respective cell spaces may be defined as the cell pitch.
As to the thickness of the above-mentioned partition walls, if each of them is set to be less than 0.05 mm when the above pitch is kept, then each opening area of the ventilation cells can increase due to the setting. However, it is not preferable that there arises such apprehension that a fragile property in strength thereof is brought about when the ceramic electronic part is situated or when the setter for burning is handled. On the other hand, if the thickness is set to be greater than 1.0 mm, each opening area of the ventilation cells decreases due to the setting. Therefore, there arises such apprehension that each function of the ventilation cells decreases relative to the oxygen supply and the gas scatter depending on the ceramic electronic part. The thickness thereof is more preferably 0.1 to 0.5 mm and still more preferably 0.1 to 0.2 mm.
Also, as to the above-mentioned cell pitch, if each cell pitch is set to be less than 0.5 mm when each thickness of the above-mentioned partition walls is kept, then the scattered gases are difficult to pass through before individual cells decrease in size. In addition, an area of each inside surface of the cells per opening ratio in a total of the ventilation cells increases, and probability of adsorption of the gases to each inside wall surface of the cells increases when the gases is scattered. The setter for burning is usually used repeatedly many times. Therefore, in the case where the gases-adsorption phenomenon occurs, it is necessary to carry out baking before the setter is used and to completely oxidize a reducing gas such as CO or the like to scatter the gases. This results in the cause of extra labor and time. A constitution requiring no process such as baking is desirable.
If the cell pitch is set to be larger than 5.0 mm, the opening area per ventilation cell increases, and there also occurs the case where there arises such apprehension that when a small type electronic part is situated, it falls from the setter for burning. The cell pitch is more preferably 0.8 to 3.0 mm and still more preferably 1.0 to 2.5 mm.
A cross-sectional shape of each ventilation cell defined by the above-mentioned cell pitch and the above-mentioned thickness of each partition wall can be easily formed by using molds configured for a honeycomb shape. For example, the cross-sectional shape may be made into an almost square. Such the cross-sectional shape may be designed to be an optional polygon such as a rectangle, lozenge, hexagon, octagon or th like.
Further, if an opening ratio, which is a ratio of the total opening area of said plurality of ventilation cells to the total area of said electronic part placement surface, is 50 to 90%, then more preferable results in view of oxygen supply ability and gas scatter ability can be obtained. The opening ratio is more preferably within a range of 60 to 90% and may be still more preferably 70 to 90%.
The setter for burning, which has the above structure according to the present invention, may be provided with a supporting member having a predetermined height in a side of the in-furnace placement surface. By providing the supporting member, the side of the in-furnace placement surface for situating the setter for burning can be provided apart from the inside of the furnace. By providing the side of the in-furnace placement surface apart from the inside of the furnace, air freely goes in and out from the side of the furnace placement surface unlike the case where the setter for burning is solid situated without providing the side of the in-furnace placement surface apart from the inside of the furnace.
Due to this, when the ceramic electronic part situated on the setter for burning is burned, oxygen amounts required for burning binders and plastic materials in the ceramic electronic part can be satisfactorily ensured from the in-furnace placement surface. By this, stay of incomplete combustion gas such as CO or the like, which affects the warp and the like of the ceramic electronic part burned, does not occur, either.
Also, by providing the supporting member to go freely air in and out from a side of the in-furnace placement surface, it is possible to ensure passages for easily passing combustion gas such as the binders, the plastic materials and the like included in the burned ceramic electronic part to be situated on the setter for burning. In this manner, by providing the supporting member on the side of the in-furnace placement surface, effects such as ensuring of the oxygen amounts and the passage for the combustion gas and the like can be obtained. In order to make the effects valid, height of the supporting member may be set to be 1 mm or more and 10 mm or less.
When the height is less than 1 mm, permeability such as the oxygen amounts, the passages for the combustion gas and the like can be insufficiently ensured. It is unnecessary that any limitation on the height of the supporting member is set from a viewpoint of ensuring of the permeability. However, it is reasonably preferable to set the upper limitation within a certain range taking it into consideration that an actual operation is carried out in the furnace. For instance, when the setter for burning is utilized, there is the case where setters for burning are piled up into a multi-stage structure at site. From a viewpoint of a valid utilization of an inside space of the furnace in such the situation in use, it is appropriate to set the height of the supporting member to be 10 mm or less and more preferably 5 mm or less.
Installing positions of the supporting member in the side of the in-furnace placement surface, for example, may be four corners Or, by forming the supporting member in a bar shape, such bar-shaped supporting members may be arranged with each a given space under a side of the in-furnace placement surface, that is, may be provided like teeth of clogs. Alternatively, the supporting member may be provided such that supporting portions are integrated with the setter for burning. In short, if gas permeability between the side of the in-furnace placement and the inside of the furnace can be ensured, it is not particularly necessary to limit the shapes and the installing positions of the supporting members.
In the setter for burning, as described above, it is desirable to use materials having a large porosity so that the oxygen supply, the gas scatter and the like can be satisfactorily ensured. However, a material having a larger porosity than required is considered to result in the case where the strength required for the setter for burning within a general operational range cannot be ensured. In the structure in which the ventilation cells are provided as through-holes as mentioned above, there arises such strong apprehension that the strength thereof decreases. It is therefore required to provide the upper limit of the porosity relative to each ventilation cell. The porosity may be 70% or less from this pint of view. On the other hand, if the porosity is less than 15%, contribution of gaps to the oxygen supply and the gas scatter becomes small. It is therefore required to increase the opening ratio in order to ensure the oxygen supply and the gas scatter to be obtained by the ventilation cells due to the contribution, and it is not desired to cause reduction in strength thereof on the contrary. The porosity is more preferably 20 to 60%.
Here, although it is effective to set the opening ratio and/or the porosity to be higher values, there arises such apprehension that the strength thereof is reduced at the same time. For this, by providing R portions each having a size of 0.1 mm or more at each crossing section between the partition walls of the ventilation cells, it is desirable to compensate for the reduction thereof in strength. In this case, the size of each R portion is provided properly by taking the thickness of each partition wall, the pitch, the opening ratio and the like into consideration.
The inventors have considered under what states the ceramic electronic part is situated on the setter for burning. As a result, it has been confirmed that the surface roughness is preferably smaller in the condition that the opening ratio is made large enough to ensure an atmosphere suitable for the above-mentioned oxygen supply and the gas scatter, whereas the surface roughness is preferably larger in the case where the above atmosphere is not ensured.
It is estimated that in the condition that if an acidic atmosphere such as the oxygen supply or the like is insufficiently ensured, then the situated state can be closer to a point contact state than to the solid situated state by increasing the surface roughness of the setter for burning and the oxygen supply circumstance is thereby improved to that extent, so that desirable results can be obtained. That is, when the above-mentioned acidic atmosphere is insufficiently kept, the magnitude of the surface roughness affects greatly.
As to the surface roughness of the surface on which the electronic part is situated and which is constituted by the partition walls of the ventilation cells, it is desirable that the maximum height (Rmax) be 1 xcexcm or more and 100 xcexcm or less and the center line average roughness (Ra) be 0.5 xcexcm or more and 50 xcexcm or less. It is more preferable that the maximum height (Rmax) be 2 xcexcm or more and 60 xcexcm or less and the center line average roughness (Ra) be 1 xcexcm or more and 10 xcexcm or less. The surface roughness may be measured on the basis of the definition and indication of surface roughness in JIS Standard B 0601.
Also, when ceramic molded products such as ceramic electronic parts are situated on the setter for burning, the ceramic molded products burned results in a burned state in accordance to the undulation of the surface on which the electronic parts are situated if the molded products are solid situated on the electronic part placement surface of the setter for burning. Therefore, the electronic part placement surface is preferably as flat as possible. From this point of view, the flatness of the electronic part placement surface is preferably 50 xcexcm or less. The flatness is more preferably 20 xcexcm or less. In this case, the flatness may be measured in accordance with the definition and indication of geometrical deviation in JIS Standard B 0621.
The side of the electronic part placement surface is preferably processed within the above-mentioned range by using the acidic atmosphere in the furnace in this manner. However, on the other hand, the side of the in-furnace placement surface does not necessarily require accuracy. When the setter for burning is directly placed on the inside of the furnace without using the supporting members, it is preferable that the ground plane be rather coarse and it is necessary that the flatness is 50 xcexcm or more and it is said to be preferable that the flatness be 500 xcexcm or more.
If the thermal expansion coefficient of the setter for burning is designed to be 1.5xc3x9710xe2x88x926/xc2x0 C. or less, for example, 0.3xc3x9710xe2x88x926 to 0.5xc3x9710xe2x88x926/xc2x0 C., heating and cooling are accelerated in burning the ceramic electronic parts as be desired. The gradient of a rise in temperature of the furnace and the gradient of the drop of temperature are increased, and thereby each operation time can be shortened.
In every one of the above-mentioned structures, the above-mentioned setter for burning may be formed of cordierite ceramics, alumina ceramics, zirconia ceramics or the like. The cordierite ceramics are characterized by the inclusion of 40 to 60% by weight of SiO2, 25 to 50% by weight of Al2O3 and 10 to 20% by weight of MgO.